In a prospective cohort of patients in the second phase of labor, the investigators measured the head-perineum distance and correlate it to the mode of delivery (spontaneous delivery - instrumental delivery - ceasarean section).

Head-perineum distance measured by transperineal ultrasound is a simple and well-known tool to help diagnose the fetal head engagement and predictive value to instrumental extraction succeeds.

It is performed in labor room and is a non-invasive test, realized on a healthy skin.

The investigators performed a systematical measure of head perineum distance at the beginning of the expulsive labor phase. The investigators made the hypothesis, based on our clinical experience, that starting expulsive labor phase with head perineum distance greater than forty five millimeters, lead to an increased risk of instrumental extraction and therefore an increase in materno-fetal morbidity and mortality.

The investigators sought to evaluate the optimal head perineum distance at the beginning of expulsive efforts.

Inclusion Criteria:

• Single pregnancy
• In labor (spontaneous or induced)
• Term superior at 37 SA
• Delivery by summit presentation

Exclusion Criteria:

• Non single pregnancy
• Suspected malformative abnormalities in the unborn fetus
• Refusal of participation
• Minor patient
• Adult patient subject to legal protection or patient deprived of liberty
• Poor French language understanding